1. Field of the Invention
The present invention relates to image formation, and more particularly, to optical systems which employ color-selective polarizing elements for forming color images.
2. Background of the Related Art
Optical projection systems in the related art use transmissive thin-film-transistor (TFT) liquid crystal display (LCD) panels. Multi-layer evaporated thin-film dichroic beamsplitters that are tilted with respect to the axis of incident light are used to create physically distinct paths, each representing the spectral power in one portion of an additive primary color band red-green-blue (RGB). An LCD in each path controls the local light transmission level of a particular primary color band. Modulated or imagery light is recombined with additional tilted isotropic coatings and full-color imagery is projected onto a front or rear projection screen.
In transmissive systems, the LCD is positioned between crossed polarizers as an approach to obtain high contrast ratios for most LC electro-optic modes. In reflective systems, where light is incident substantially normal to the LCD panel, the analogous configuration is to position a polarizing beamsplitter (PBS) directly in front of the panel, as described in a parent application, incorporated by reference above.
One type of polarization beam splitter is a tilted thin-film stack with four ports, which reflects or transmits a light spectrum based on its polarization. A PBS ideally functions as a broad-band reflector for a light spectrum polarized along one axis, and as a transmitter for a light spectrum polarized along an orthogonal axis. A dichroic beamsplitter ideally reflects or transmits a light spectrum based only on the wavelength of the light.
A full-color split-path projector may use reflective LCD panels, with dichroic beamsplitters for creating three color paths, a polarizing beamsplitter for each reflective LCD panel, and additional optics for recombining the imagery before the projection lens. Such implementations are cumbersome and expensive.
An alternative is to use a single polarizing beamsplitter, followed by a Philips prism for separating and combining the three color paths. In this architecture, the color splitting/combining structure is effectively positioned between the polarizer and an analyzer. A benefit from a component count standpoint is that the three color paths share the same PBS.
However, high contrast ratio mandates that the Philips prism preserves the input state of polarization for each color path so that light efficiently exits the input port. This condition must be maintained such that the contrast ratio averaged over the f-number of the system is 100:1, and ideally exceeds 200:1. Clean up polarizers, and in some cases additional polarization optics, such as quarter-wave plates, between the LCD panel and prism can then be used to improve the contrast ratio.
In the Philips prism, red and blue are first reflected by a dichroic coating and then total internally reflected (TIR) before impinging on an LCD. Polarization modulated light then returns along the same path. The spectral characteristics of the dichroic coating are strongly dependent on incidence angle, creating a significant cross-talk between color channels. To help overcome problems with cross-talk, a xe2x80x9cdouble notchxe2x80x9d filter (DNF) is frequently inserted which substantially blocks interband light, such as true cyan and true yellow. The DNF is also a multi-layer coating, but because it is used at near normal incidence, it is less sensitive to changes in incidence angle. Nonetheless, when averaged over the f-number of the system, the density of light at the notch is reduced.
Accordingly, related art three-panel reflective projection systems use a PBS, a DNF, and a Philips prism, each of which consists of three prisms, two of which have dichroic mirror coatings. To achieve the performance of transmissive systems using reflective LCD panels, an architecture is required that reduces the complexity and cost, while increasing contrast ratio and throughput.
Multi-layer thin-film coatings are used in the related art for manipulating color in projection display systems. This technology is well matched to the high efficiency and high power handling requirements of projection. Moreover, the steep transition slopes desired to maximize luminance, while meeting color coordinate standards, can be achieved. However, tilted isotropic coatings can degrade polarization quality, particularly in low f-number systems. In LCDs, polarization must be accurately preserved in order to achieve low dark state leakage. Furthermore, dichroic mirrors have an angle sensitive half-power wavelength that shifts substantially with incidence angle.
In order to create physically distinct color paths using a dichroic mirror, the layers are often substantially tilted with respect to the axis of incident light. This significantly increases the spectral shift covered by small angular excursions with respect to the bias angle. At a worst case bias angle of 45xc2x0, the wavelength (spectral) shift is approximately linear with angular change.
Fundamentally, the apparent thickness of each layer in a thin-film stack is reduced with an off-normal incidence angle, resulting in a blue shift of the spectrum. When a bias angle is present, both blue and red shifts are present in the plane of incidence. Such angle sensitivity can limit the f-number in color management systems and, in particular, LCD projectors.
Reflective silicon display panels are readily known in the related art. The most common reflective silicon display panels are VLSI-based active-matrix panels that are processed to have a high or flat fill factor, and high visible reflectivity. Alternatively, polysilicon panels can be made to function as reflective displays. In VLSI-based panels, a thin liquid crystal film is sandwiched between the silicon chip and a cover glass coated with a transparent conductor, typically indium tin oxide (ITO). The liquid crystal can be either a nematic or smectic material, both of which are well documented in the art. The liquid crystal is an anisotropic medium, which responds to an electric field by changing its orientation. This in-turn changes the polarization state of light propagating through the liquid crystal.
FIGS. 1(a) and 1(b) illustrate related art reflective display architectures where light having a single polarization state is introduced. As shown in FIG. 1(a), light enters a polarizing beamsplitter (PBS) 10 through a first port 12, and is reflected out a second port 14 towards a reflective LCD panel 20. The LCD panel 20 reflects the light back through the second port 14 and the PBS 10, where the light exits via a third port 16. In FIG. 1b, the light enters the PBS 10 through the third port 16, travels through the PBS 10, and exits through the second port 14. The LCD panel 20 reflects the light back through the second port 14, where the light is reflected by the PBS 10 and exits via the first port 12.
The polarization state of reflected light is locally modulated via the voltage dependent distribution of the LC molecules at each pixel of the LCD panel 20. This polarization encoded imagery is converted to an actual gray shade image using an analyzing polarizer. In the retroreflecting arrangements shown in FIGS. 1(a) and 1(b), light is introduced and analyzed using the PBS 10. The PBS 10 effectively positions the LCD panel 20 between crossed polarizers , and also directs light through the system and ultimately to projection lenses.
An object of the present invention is to substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
Another object of the present invention is to reduce the complexity and cost while increasing contrast ratio and throughput of reflective LCD systems.
Still another object of the present invention is to reduce the f-number in color management systems while maintaining contrast ratio.
It is recognized, in commonly owned U.S. Pat. No. 5,751,384, the contents of which are incorporated herein by reference in their entirety, that retarder stacks can be used in conjunction with neutral polarization splitters to create separate color paths and to combine separate color paths to form a single path. There are inherent benefits of this technology, particularly in terms of angle sensitivity.
The present invention can be achieved, in whole or in part, by an optical system including an input retarder that transforms a first spectrum of input light from a light source along a first polarization state, and transforms a second spectrum of the input light from the light source along a second polarization state different than the first polarization state, and a beam splitting unit, optically coupled to the input retarder, and including a first beamsplitter that transmits the first spectrum as a transmitted spectrum, and that reflects the second spectrum as a reflected spectrum.
The present invention can also be achieved, in whole or in part, by a system that divides input light from a light source into component color bands, red, green, and blue by making the colors travel different physical paths. At least two of these paths are created using stacked retardation films and a polarization beamsplitter (PBS). By creating distinct paths, each color band can be independently processed. According to preferred embodiments of the present invention, such processed light can be combined to form a single path, again using a retarder stack (RS) and a PBS. The present invention is particularly suited to projectors using reflective liquid crystal on silicon display panels.
The present invention can also be achieved, in whole or in part, by merging color and polarization management to produce a split-path projector, based on reflective display panels, that is simple in construction. Retarder stack (RS) components create orthogonally polarized primary colors from a polarized input. In this exemplary embodiment, a PBS functions as a color splitter, allowing all four ports of the PBS to be utilized. The port containing the subtractive primary band can be further split using a dichroic beamsplitter. In one embodiment of the reflective architecture, all three paths are recombined and analyzed by the input PBS. Full-color imagery can exit the previously unused fourth port of the PBS. A full-color projector according to the present invention would therefore require only one PBS coating and one dichroic color splitter coating, along with one or two retarder stacks.
The present invention can further be achieved, in whole or in part, by improving polarization management associated with color optical systems to provide a high contrast ratio. The reduced angle sensitivity exhibited by the retarder stacks of the present invention, relative to dichroic splitters, also results in the projectors of the present invention having high contrast ratios.
The present invention can also be achieved, in whole or in part, by providing a wide color gamut with minimal hardware. An aspect of the invention is the recognition that an exit retarder stack, used to manage light leakages from the PBS, can also be used to generate inter-band notch filtering operations. This eliminates the need for an auxiliary notch filter, which is used frequently with related art systems that utilize the Philips prism. Using different input and exit retarder stacks, inter-primary light, such as that produced by a metal halide lamp, can be diminished or eliminated by the exit clean up polarizer. This eliminates the need for a separate double notch filter (DNF).
The present invention can still further be achieved, in whole or in part, by providing an optical system, such as a reflective LCD projector, that exhibits high overall throughput, or brightness. This is achieved by manipulating color bands with stacks of lossless polymer retarder films, by providing refractive index matching between the retarder films, and by minimizing the number of lossy polarizers. It is further accomplished by eliminating the need for an auxiliary filter for eliminating inter-primary light.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.